Add save_cookies/load_cookies to python CookieJar

Some wired behavior discovered in LWPCookieJar,
(hopely they dont cause any more problem):
-- on saving cookies
throw err if .domain is None,only accept ""
throw error if .expires is None.
(though CookieJar works fine with None, many examples use None)
-- on loading cookies
LWP only set domain_specified=True for cookies with domain start with dot.